package com.company.gxcpc2018;

import java.util.Scanner;
import java.util.stream.Stream;

public class Gxcpc4 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int t = sc.nextInt();
        while (t-- > 0) {
            int n = sc.nextInt();
            sc.nextLine();
            String[] words = Stream.generate(sc::nextLine).limit(n).toArray(String[]::new);
            String name = "";
            int i = 1;
            while (true) {
                boolean flag = true;
                String s = intToWord(i);
                for (String word : words) {
                    if (word.contains(s)) {
                        flag = false;
                        break;
                    }
                }
                if (flag) {
                    System.out.println(s);
                    break;
                }
                i++;
            }
        }
    }

    public static String intToWord(int num) {
        num--;
        StringBuilder str = new StringBuilder();
        do {
            if (str.length() > 0) {
                num--;
            }
            str.insert(0, intToChar(num % 26));
            num = (num - (num % 26)) / 26;
        } while (num > 0);
        return str.toString();
    }

    public static char intToChar(int num) {
        return (char) ('a' + num);
    }
}
